git-gui: Use mixed path for docs on Cygwin.

The Firefox browser requires that a URL use / to delimit directories.
This is instead of \, as \ gets escaped by the browser into its hex
escape code and then relative URLs are incorrectly resolved, Firefox
no longer sees the directories for what they are.  Since we are
handing the browser a true URL, we better use the standard / for
directories.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -5336,7 +5336,7 @@ set doc_path [file dirname [gitexec]]
 set doc_path [file join $doc_path Documentation index.html]
 
 if {[is_Cygwin]} {
-	set doc_path [exec cygpath --windows $doc_path]
+	set doc_path [exec cygpath --mixed $doc_path]
 }
 
 if {$browser eq {}} {